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Relative contributions of gas exchange and gravity related nitrogen gradients on the slope of phase iii of single breath nitrogen test



Relative contributions of gas exchange and gravity related nitrogen gradients on the slope of phase iii of single breath nitrogen test



Lung 160(1): 37-44



Evidence for the effect of gas exchange on the slope of phase III was previously obtained using a reversed gradients technique (SB-R). With this technique, the subject inspires a vital capacity of room air after a complete washout with 100% O2. Tracings with this test always had a flatter slope of phase III than SB-N2 [single breath N2 test]. This study revealed similar results, with the slopes of phase III of 0.44 .+-. 0.32% N2/l for SB-R and 1.00 .+-. 0.36% N2/l for SB-N2 (P < 0.01). Theoretically, SB-R should be the mirror image of SB-N2. To verify that this difference truly represented gas exchange, and is not the consequence of increased overall N2 concentration, and also to quantitate the influence of gas exchange, 10 normal human volunteers were studied with SB-N2, SB-R and 2 modified reverse gradients tests. The 2-modified tests were devised to increase the overall N2 concentration (SB-M) and to increase intrapulmonary N2 gradients resulting from different ventilation to initial volume (.DELTA.V/Vo) ratios (SB-R 100). SB-M is a 2-min washout with 79% O2 and 21% N2 followed by a 100% N2 vital capacity. SB-R100 is a 2-min washout with 100% O2 followed by a 100% N2 vital capacity. SB-M gave a slope of phase III similar to SB-R (0.54-0.25% N2/l and 0.44 .+-. 0.32% N2/l, respectively). Apparently, increasing overall N2 concentration is not responsible for the observed difference between SB-N2 and SB-R. In SB-R100 the slope of phase III increased to 0.71 .+-. 0.34% N2/l (P < 0.01 with SB-R values), but it remained less steep than in the standard SB-N2 test (1.00 .+-. 0.36% N2/l). This flatter slope, despite larger .DELTA.V/Vo-induced gradients, supports the role of gas exchange on the slope of phase III. From these data, it was calculated that gas exchange may contribute as much as 28% of the slope of phase III.

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Accession: 006303812

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